Update ReadMe.md
Replaces Verifies that by Checks that within the INumberFluentAssertion methods comments.
Includes the new CONTRIBUTING.md file.
Updates the Contributing.md file.
Indicates that Apache 2.0 license header is not mandatory for unit test
files.
Improves both the Readme and CONTRIBUTING md files.
Emphases the Dev DoD within the CONTRIBUTING.md
Merge branch 'master' of https://github.com/malat/NFluent

Conflicts:
	NFluent/INumberFluentAssertion.cs
	NFluent/NumberFluentAssertion.cs